1. Field of the Invention
This invention is concerned with a method of and apparatus for controlling gray scale or print intensity in a matrix printer using constant amplitude write pulses. More particularly, this invention relates to a matrix printer implemented method and arrangement for controlling gray scale in charge sensitive media by varying, as a function of the desired intensity of a pixel to be printed, the total write time that a constant amplitude pulse is applied to a print electrode and when it is so applied over the course of one complete write cycle.
2. Description of the Prior Art
In all non-impact printers that utilize electrosensitive recording mediums, it is possible to effect printing by applying sufficient voltage over a predetermined time period to the write styli of the particular printer involved. This ability to "write" on an electrosensitive recording medium is chiefly a function of the amplitude and/or duration of the voltage pulses applied to the print styli or electrodes. All known printers of this type rely only on amplitude variations to effect printing, since variations in the relative difference between the speed at which the recording medium is moved and the speed at which a write cycle is completed, including the point or points within a write cycle at which a pixel is started and completed, could cause registration errors that adversely affect print quality unless compensated for. Such compensation is either more expensive than the use of write pulse amplitude control or requires that the recording medium be slowed appreciably or stopped during the write cycle.
Except for this registration problem and the cost or performance penalty required to compensate therefor, the time variation method would find widespread favor as it would allow the use of fully saturated devices and also avoid the need to utilize variable drive voltages for the write electrodes. This, in turn, would result in increased operating efficiency, lower power consumption, and reduced heat buildup and dissipation problems.
Prior art attempts to utilize time-varied, constant amplitude pulses are known. One such typical prior art arrangement is described in U.S. Pat. No. 3,441,946 which issued to Rudy. The Rudy circuitry is intended to supply write signals in a single stylus recorder that uses electrosensitive paper. More particularly, this write control circuitry is adapted to vary the frequency of write pulses as a function of the speed at which the recording medium is driven under the print stylus. Thus, the designer is able to compensate for differing recording conditions and speeds and control the degree to which the medium is marked. U.S. Pat. No. 3,553,718, issued to Schierhorst et al, describes another prior art printer wherein time-based control of a write electrode is employed. In Schierhorst et al, a constant amplitude current pulse is sequentially applied to a plurality of write styli, the duration of said pulse being proportional to the signal to be recorded so that the shading of the recording represents the amplitude of the signal.
U.S. Pat. No. 3,613,103 to Harris is also of interest for its teaching of an electrolytic recorder wherein an analog signal to be recorded is converted to a series of pulses with rises spaced in inverse proportion to the amplitude of the analog signal. Control circuitry therefor is adapted to create this pulse series and thereafter utilize the same to trigger a pulse generator whose output is a corresponding series of constant amplitude write pulses having a spatial density that is proportional to the original signal amplitude. U.S. Pat. No. 3,846,801 to DuMont et al is yet another prior art effort that utilizes time-based printing control. Specifically, DuMont et al describes an arrangement for sequentially activating a multiplicity of juxtaposed writing electrodes. To produce the sequential operation of the write electrodes, a time-division demultiplexer is used to distribute time subdivisions of the signal to the respective electrodes. The DuMont et al control arrangement is further adapted so that the write electrodes are activated for just one pulse each during any one clock period.
Unfortunately, none of these prior art write electrode control arrangements are satisfactory or suitable for use where multiple electrodes are to be ued and the recording medium is moved in the printer during the actual printing operation. They would either cause unacceptable print registration errors or simply not be adaptable to the requirements of current matrix printers that use constantly moving electrosensitive recording mediums.